Tunnel to replace Alaskan Way Viaduct

By Richard High15 January 2009

Washington State Governor Chris Gregoire, Seattle Mayor Greg Nickels and King County Executive Ron Sims have signed an agreement to replace the central waterfront portion of the Alaskan Way Viaduct and Seawall with a US$ 1.9 billion tunnel.

While the 1.7 mile-long (2.74 km) bored tunnel - expected to be a stacked tube with two lanes in each direction, will cost just under US$ 2 billion, the total cost of the project is US$ 4.24 billion. This includes the entire viaduct corridor - over 2 miles (3.22 km), a surface street, a new seawall in the central waterfront, utility relocation, city streets and transit pathways, transit infrastructure, construction transit service and all prior program expenditures

The state, city, county and Port of Seattle will each fund part of the US$ 4.24 billion needed to the complete project. The state's contribution, US 2.82 billion, will pay for Alaskan Way surface street in this area - but not the promenade. That portion is funded by the City of Seattle.

The county will contribute US$ 190 million for improvements to the Metro bus service and city streets, while the city will pay US$ 930 million for the Alaskan Way promenade and a First Avenue Streetcar.

The Port of Seattle, currently awaiting its Comission's review and approval, is expected to contribution US$ 300 milllion to the project.

Replacement

According to the Washington State Department of Transport (WSDOT) website the Alaskan Way Viaduct section of SR 99 is 55 years old. It is main north-south route through the city, carrying 20 to 25% of the traffic travelling through the downtown area. However, time, daily wear and tear, salty marine air, and some sizeable earthquakes have taken their toll on the structure.

Studies in the mid-1990s showed the viaduct was nearing the end of its useful life, signalled by crumbling concrete, exposed rebar, cracking concrete, weakening column connections, and deteriorating railings.

In 2001 a team of design and seismic experts began work to determine whether it was feasible and cost-effective to strengthen the viaduct by retrofitting it. In the midst of this investigation, the 6.8 magnitude Nisqually earthquake shook the Puget Sound region. The earthquake damaged the viaduct, forcing the WSDOT to temporarily shut it down.

Post-earthquake inspections of the viaduct revealed that while it survived earthquake damage to the viaduct's joints and columns had further weakening the structure, revealing its severe vulnerability. It was therefore concluded by WSDOT that it was not cost-effective to fully retrofit the majority of the viaduct; rather, the viaduct would need to be rebuilt or replaced.

Seawall

Shortly after the Nisqually earthquake, a 100 ft-long by 10 ft-wide (30.5 m-long by 3.05 m-wide) section of the Alaskan Way surface street settled, raising concerns about the condition of the Alaskan Way Seawall.

The seawall holds the soil in place along Seattle's waterfront. The seawall also holds the Alaskan Way surface street and many utilities in place. The viaduct's foundations are embedded in the soil held back by the seawall. If the seawall were to fail, sections of the viaduct, the Alaskan Way surface street, and adjacent structures and utilities could collapse or become unsafe.

Investigations showed the seawall's condition was worse than expected and it needed replacing. The seawall continues to deteriorate despite regular maintenance by the City of Seattle. Soils underneath the roadway moved and liquefied during the Nisqually earthquake.

In addition, marine organisms called gribbles have been eating away at the timbers that support the seawall. Inspections have shown that substantial portions of the seawall's timber support structures have been weakened or destroyed by gribbles.

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